FIG. 1 is a schematic cross sectional view of an aircraft wing root joint comprising an aircraft wing box 1 attached to a centre wing box 10. The centre wing box 10 comprises a rib 11 connecting upper and lower root joint fittings 2, 3. The aircraft wing box 1 comprises upper and lower laminar composite covers 4, 5 connected by a spar 6 which extends between them.
The covers 4, 5 have inner and outer mould lines 4a, 5a, 4b, 5b. The outer mould lines 4b, 5b form external aerodynamic surfaces of the wing box 1, while the inner mould lines 4a, 5a, form internal surfaces of the wing box 1 opposite the outer mould lines 4b, 5b. 
The spar 6 is shown in longitudinal section in FIG. 1. The spar 6 is C-shaped in transverse section with upper and lower spar flanges 7, 8 which extend from a web 9 and engage the inner mould lines 4a, 5a of the covers 4, 5. The spar flanges 7, 8 terminate outboard of the inboard edge of the spar web 9 to leave spar flange cutback areas where the covers 4, 5 overhang the flanges 7, 8 in an inboard span-wise direction towards the root joint fittings 2, 3.
A problem with manufacturing such a joint is that the thickness of the laminar composite covers 4, 5 cannot be controlled accurately due to the method of manufacturing the laminate and the properties of the composite material. As a result the covers 4, 5 may not precisely meet the root joint fittings 2, 3 resulting in either a gap or a clash. This problem can be solved by bending the covers 4, 5 (where they overhang the spar flanges 7, 8) either towards the fittings (to close a gap) or away from the fittings (to avoid a clash). However this will induce stress in the covers 4, 5 and as a result the covers must be made thicker and heavier to cope with this stress. Also it may be difficult to bend the covers sufficiently, particularly near the cover/spar interface shown in FIG. 1.
The thickness of the upper cover 4 (i.e. the distance between the inner mould line 4a and the outer mould line 4b) is particularly critical as it is its outer mould line 4b (rather than its inner mould line 4a) which engages the root joint fitting 2. Clearly, any increase or decrease in the thickness of the upper cover 4 will affect the alignment between the upper cover 4 and the upper root joint fitting 2 and/or the alignment between the lower cover 5 and the lower root joint fitting 3. Conversely, as it is the inner mould line 5a of the lower cover 5 which engages the lower root joint fitting 3 (rather than its outer mould line 5b), the thickness of the lower cover does not affect the alignment between the lower root joint fitting 3 and the lower cover 5, nor does the thickness of the lower cover affect the alignment between the upper cover 4 and the upper root joint fitting 2. However, the thickness of the lower cover is critical for the attachment of a buttstrap 12 which connects the lower cover OML 5b to a lower cover 13 of the centre wing box 10. Therefore, any thickness variation in the manufacturing process of both covers must be carefully managed.